1. Field of the Invention
This invention relates to a sulfide phosphor which emits green to orange light with long after-glow when excited with an electron beam, a long after-glow white emitting phosphor containing the yellow emitting sulfide phosphor as a yellow emitting constituent, and an electron excited fluorescent display device using the green to orange emitting phosphor or the white emitting phosphor.
2. Description of the Prior Art
Recently, electron excited fluorescent display devices such as cathode ray tube (CRT) and low-velocity electron excited fluorescent display device (The term "electron excited fluorescent display devices" referred to in this specification should be interpreted as CRT's or low-velocity electron excited fluorescent display devices.) are used for various purposes, and there is desired a phosphor exhibiting good after-glow after supply of excitation energy thereto is stopped. For example, in terminal display units of a computer system for displaying fine characters and figures, display units of an aircraft control system, and the like, it is desired to use a CRT exhibiting high resolution. As an effective method of improving the resolution of the CRT, it is known to reduce the frame frequency of the CRT. Namely, by reducing the frame frequency of approximately 55 Hz in ordinary CRTs such as television CRTs to approximately 30 Hz, it is possible to expand the signal frequency band by approximately two times that of ordinary CRTs or to set the image frequency to approximately half of the image frequency of ordinary CRTs, thereby to increase the resolution. The reason why the resolution can be increased by reducing the frame frequency of the CRT is that the image frequency band of a CRT drive circuit is determined by the product of the frame frequency and the signal frequency band.
The fluorescent screen of the high resolution CRT as described above should be constituted by a phosphor exhibiting long after-glow. This is because, if the fluorescent screen of the CRT is constituted by a short after-glow phosphor, the image displayed on the fluorescent screen flickers undesirably due to a low scanning speed on the fluorescent screen. In general, it is necessary for the phosphor constituting the fluorescent screen of the high resolution CRT to exhibit an after-glow period several tens of times to several hundreds of times longer than the after-glow period of the short after-glow phosphor which constitutes the fluorescent screen of the ordinary CRTs. The term "after-glow period" as used herein means the time required for the emission luminance to decrease to 10% of the emission luminance under excitation after the excitation of the phosphor is stopped, i.e. the 10% luminance after-glow period.
As the long after-glow phosphors usable in the high resolution CRT, there have heretofore been known a manganese and arsenic activated zinc silicate green emitting phosphor (Zn.sub.2 SiO.sub.4 :Mn,As), a manganese activated potassium magnesium fluoride orange emitting phosphor (KMgF.sub.3 :Mn), a lead and manganese activated calcium silicate orange emitting phosphor (CaSiO.sub.3 :Pb,Mn), a manganese activated magnesium fluoride red emitting phosphor (MgF.sub.2 :Mn), a manganese activated zinc magnesium orthophosphate red emitting phosphor [(Zn,Mg).sub.3 (PO.sub.4).sub.2 :Mn], and the like. However, these known phosphors exhibit only limited emission color and after-glow period characteristics intrinsic to the respective phosphors, and the coating characteristics thereof are not satisfactory. On the other hand, as a wide variety of high resolution CRTs are recently required, a need exists for various long after-glow phosphors exhibiting various emission color tones, emission of high luminance, and a wide range of after-glow periods as desired. Particularly, since a phosphor emitting yellow light with long after-glow is necessary to black-and-white display CRTs, there has been a strong desire to develop such a phosphor. Further, a long after-glow green emitting phosphor and a long after-glow orange emitting phosphor are necessary for use in monochrome display devices.
However, there has not been known any single phosphor emitting white light with long after-glow. Further, since a long after-glow blue emitting phosphor and a long after-glow yellow emitting phosphor have not heretofore been known, a long after-glow white emitting phosphor obtained by mixing them does not exist. Another approach to obtaining white emission is to mix three phosphors as the red emitting component, the green emitting component and the blue emitting component. However, a long after-glow blue emitting phosphor has not been known and, in addition, since the long after-glow red emitting phosphor and the long after-glow green emitting phosphor exhibit after-glow characteristics curves different from each other, color drift occurs in the after-glow. Further, color shading readily occurs when many phosphors exhibiting emission colors different from one another are mixed. For these reasons, it is not desirable to obtain the long after-glow white emitting phosphor by mixing the three color component phosphors exhibiting emission colors different from one another.
As described above, there has not heretofore been known any phosphor emitting white light with long after-glow, and consecuently there exists no black-and-white television CRT for display that is provided with a fluorescent screen comprising the phosphor emitting white light with long after-glow and which can display an image at high resolution.
On the other hand, since the visual sensitivity of the eye of human is higher to light having a wavelength within the green region, green light can be felt as being highly luminous compared with the other emission colors even when the emission energy is the same, and does not cause the eye to be much fatigued even when the viewing time is long. Therefore, a display CRT (hereinafter simply referred to as "CRT") emitting green light and exhibiting high resolution is widely used.
As the long after-glow green emitting phosphor to be used in such a CRT, a manganese and arsenic activated zinc silicate phosphor (P39 phosphor) or the like has heretofore been known. The P39 phosphor is the sole phosphor available at present as the long after-glow green emitting phosphor put into practice from the viewpoints of both emission luminance and the after-glow period, and is used in large amounts. However, since the P39 phosphor contains arsenic, it presents a very real problem with regard to pollution. Accordingly, a need exists for a CRT using a long after-glow green emitting phosphor containing no pollutant. Further, the P39 phosphor obtained according to the preparation method heretofore used is in the form of finely-divided grains having an average grain size (single grain size) within the range of 2.mu. to 3.mu.. Even larger grains obtained by the preparation method have a grain size of 5.mu. or less, and the grain size distribution achieved by the method is wide. On the other hand, from the viewpoints of coating characteristics, emission efficiency and the like, phosphor grains having a median of grain size within the range of about 6.mu. to 12.mu. and exhibiting narrow grain size distribution are generally used as the phosphor coated on the fluorescent screen. Because of the above and also because of the crystal shape, the coating characteristics of the P39 phosphor are not good. The P39 phosphor is disadvantageous also in that the yield of the phosphor product markedly drops when the phosphor grains are classified to obtain grains of uniform grain size, and the phosphor coat formed on the fluorescent screen becomes uneven due to difference in grain size when the phosphor is mixed with another phosphor.
A CRT emitting orange light and exhibiting high resolution is also used widely since it does not cause the eye to be much fatigued even when viewed for a long period.
As the long after-glow orange emitting phosphor to be practically used in such a CRT, a phosphor mixture of the P39 phosphor with a manganese activated zinc phosphate (P27 phosphor) emitting red light has heretofore been used. However, since the phosphor used is a mixture of two phosphors exhibiting emission colors different from each other, it develops color shading when resolution is set high and is not suitable for practical use. Further, the luminance of emission of this phosphor mixture is insufficient. On the other hand, as a single phosphor emitting orange light with longer after-glow, a lead and manganese activated calcium silicate phosphor (P25 phosphor) is known. However, the luminance of emission of the P25 phosphor is too low to be suitable for practical use, and the emission color thereof scarcely changes with the amount of activator contained therein, or the like. Thus, the P25 phosphor cannot exhibit various emission colors and is not suitable for use in CRTs.
Further, as a CRT adapted to a wide variety of applications is recently demanded, it has been desired to develop a CRT provided with a fluorescent screen comprising a long after-glow orange emitting phosphor the emission color tone of which can be freely selected from the range between orange near to red and orange near to yellow, and which exibits emission of high luminance. On the other hand, the fluorescent screen of a high resolution CRT is required to exhibit markedly high uniformity from the viewpoint of the CRT functions compared with the fluorescent screen of the conventional color television CRT or the like. However, the grain shape and the grain size distribution of the aforesaid P25 phosphor, P27 phosphor and P39 phosphor are not suitable for forming a uniform fluorescent screen. Therefore, these phosphors make the yield in production of CRTs using them very low, and are very disadvantageous from the industrial viewpoint.